Many metal alloys when melted react with one or more atmospheric gases to yield undesirable reaction products. Magnesium for example burns in the presence of atmospheric oxygen. Aluminum reacts with atmospheric oxygen to form a dross layer of aluminum oxide. Both metals may be melted in closed, indirectly fired or resistance furnaces. These furnaces are open to the atmosphere only for short periods of time to withdraw portions of the melt body for casting.
The standard techniques in the art for protecting magnesium and aluminum melts in these closed furnaces have various drawbacks. Magnesium melts are most often protected by a blanketing atmosphere containing sulfur hexafluoride, SF6. Sulfur hexafluoride is a greenhouse gas and is being phased out of use in the United States due to government regulations. An alternative blanketing gas is sulfur dioxide, SO2. Sulfur dioxide is a toxic gas with government mandated exposure limitations. Sulfur dioxide also contributes to acid rain and particulate pollution and is therefore subject to environmental regulations. Sulfur dioxide is consequently difficult to use in magnesium melting furnaces in practice. Inert atmospheres such as Argon gas blankets have been attempted for Magnesium melts. The difficulty with applying an inert atmosphere is that Magnesium coats the surfaces of the furnace over time. Because the furnace must be opened to remove Magnesium, e.g. with a ladle, the argon atmosphere is compromised. The Magnesium on some surfaces then becomes exposed to air and burns violently. One solution has been the replacement of sulfur hexafluoride with other fluorine compounds having more environmentally favorable properties. One such replacement compound is C3F7C(O)C2F5, sold by 3M under the name NOVEK 612™. This replacement chemical forms a dense cover gas analogous to sulfur hexafluoride. NOVEK 612™ is limited in that it should not be used with the standard dry air atmosphere but should be supplied to the furnace in a carrier gas of 80-95% carbon dioxide with the balance being dry air. NOVEK 612™ is manufactured exclusively by 3M and consequently the supply is limited. There is thus a need for a more readily accessible alternative having multiple sources.
In closed, indirectly fired or resistance furnaces used for Magnesium, there is therefore a need for an effective alternative to the presently available inert gas protection techniques.
Aluminum melts are both protected prior/during melting and treated after melting to reduce atmospheric contaminants, particularly hydrogen and oxygen. Oxygen reacts to form a dross of Aluminum Oxide. Hydrogen derives from atmospheric water vapor and dissolves into molten Aluminum. Protection from atmospheric gases prior to and during melting may include purging the empty furnace with Nitrogen or Argon gas and maintaining the enclosed furnace space with the same inert atmosphere. See, e.g., U.S. Pat. No. 5,211,744. This is generally followed by a degassing flux such as argon and/or nitrogen bubbled through the aluminum to remove hydrogen and other impurities. The protective atmosphere of inert gas may be maintained by the same gas injections systems used for degassing. Active degassing has in the past been performed using a source of Chlorine such as Hexachloroethane tablets. Active degassing agents have become difficult to use in practice due to environmental and safety concerns. A drawback to using inert gas atmospheres is excessive cost in part because of the volume of gas required to effectively reduce dissolved hydrogen levels and dross formation.
In closed, indirectly fired or resistance furnaces used for Aluminum, there is therefore a need for an effective alternative to the presently available protection techniques.
Existing solutions to inerting of molten metal surfaces fall into two categories: liquid cryogen delivering and gaseous atmosphere blankets. Delivering the inerting cryogen to cover the surface of the molten metal is a superior technique in certain contexts. Such technologies are described in:
U.S. Pat. No. 6,491,863 B2, Stewart C. Jepson, Method and Apparatus for Efficient Utilization of a Cryogen for Inert Cover in Metals Melting Furnaces, Dec. 10, 2002.
U.S. Pat. No. 4,806,156, Sara H. Anderson, Noel F. Lutgen, Process for the Production of a Bath of Molten Metal or Alloys, Feb. 21, 1989.
U.S. Pat. No. 4,848,751, Noel F. Lutgen, Sara Hornby-Anderson, Lance for Discharging Liquid Nitrogen or Liquid Argon into a Furnace Throughout the Production of Molten Metal, Jul. 18, 1989.
Gas blanket protection is also available commercially through offers such as the PRAXAIR™ Laminar Barrier Inerting (LBI) technology and AIR PRODUCTS™ swirl cone technology.
Both categories of solution use costly inert gas, generally Argon. Thus there is a continuing need to devise new methodologies for delivering inerting gas that reduce gas utilization while maintaining the requisite degree of inertion, generally measured as a target maximum oxygen level at the molten metal surface.